The compensation of intersymbol interference in a recording system is carried out in an optical disc by providing an equalizer in a signal readout circuit, so that waveform distortion generated in a signal transmitted through the recording and readout system is removed. The equalizer is set to provide an output signal which is a readout signal having an appropriate partial-response (PR) equalizing property, so that a low error rate is obtained even in the high density storage.
In the partial-response equalization, an amount of intersymbol interference in a signal is appropriately controlled to limit an frequency band for signals in place of artificially making an amplitude-decision of a multiple-value. This is used to lower a noise power in a signal for the purpose of suppressing the occurrence of errors due to noise without emphasizing of unnecessary frequency components. In the partial-response equalization, when PR (1,1) equalization is used, a binary sequence is artificially equalized to be a three-level signal to reduce a required frequency band for signals by approximately half.
In the partial-response equalization, there is regularity for a multiple-level signal which is obtained by making an artificial decision of a multiple-value for a binary sequence. For instance, when PR (1,1) equalization is used, a signal equalized to be a three-level signal by +1, 0 and -1 has a sequence in which a readout amplitude is +1 or 0 immediately after a readout amplitude of +1 is obtained. Further, the minimum interval by which a readout amplitude of 0 is obtained is limited, when there is a limitation for the minimum intervals between transitions in recording codes.
In order to correct noise-caused errors in making decision of a multiple-level signal, a maximum-likelihood (ML) sequence detection system in which the above described regularity is used for constraint condition is used to select a signal sequence which is most coincident to a readout signal from signal sequences meeting the constraint condition. For this purpose, the Viterbi algorithm is used. The detection method which is obtained by combining the partial-response equalization and the maximum-likelihood sequence detection system is called PRML (partial-response maximum-likelihood). The PRML system will be explained in detail later.
In an optical recording or a magneto-optical recording, generally, the storage of information is realized by changing a pit configuration formed on a recording medium, or changing the direction of magnetization on a recording medium. As a result, non-linearity is generated in input and output characteristics, under the high recording density.
In a conventional equalizer, there is a disadvantage in that non-linear distortion is not removed from a signal, because it is composed of linearly operating elements such as delay circuits, coefficient-multipliers, and adders (to be explained in detail later). Therefore, when ordinary coefficient controlling procedures are adopted, an equalizing power is remarkably reduced due to the influence of nonlinearity. When distortion depending on a signal sequence is mixed as noise into an equalized signal, the distortion component prevents a maximum-likelihood sequence detection from being carried out in a normal manner to deteriorate detection characteristics.